Piezoelectric transducer and method for producing same



Feb. 6, 1951 A man 2,540,194

PIEzoELEcTkIc sDucER AND mamon Fon Pao ucnic sm:

ALEXANDER ELLETT IN1/mma HIS AGENT Feb. 6, 1951 A. ELLETT 2,540,194

PIEZGELECTRIC TRANSDUCER AND METHOD FOR PRODUCINGV SAIE A Original Filed Dec. 26. 1947 2 Shoots-Sheet 2 (las n 'ff/6.6

ALEXANDER ELLETT IN1/Ewan.

HIS AGENT Patented Feb. 6, 1951 PIEZOELECTBIC TEANSDUCER AND METHOD FOR PRODUCIN G SAME Alexander Elle,

Zenith Illinois Utilinal 793.892. Divided and 1950. Serial No. 159,620

6 Claims. (Cl. 171-327) This application is a division of the copending application of Robert Adler, Serial No. 793,892, filed December 26. 1947, assigned to the present assignee. and relates to piezo-electric transducers and to methods for producing such transducers. It is a primary object of the invention to provide an improved artiiicial transducer having permanent piezo-electric properties and to provide an improved method of producing such a transducer.

The use of polycrystalline aggregate such as barium titanate or barium strontium titanate, bonded with a ceramic binder, in the production of artificial piezo-electric transducers is specifically disclosed and claimed in the copending application of Walter L. Cherry, Jr., Serial No. 770,163, filed August 22, 1947, for Piezo-Electric Transducers. and which application is assigned to the same assignee as the present application.

Such artiiicial piezo-electric transducers as are specifically disclosed in the aforementioned copending Cherry application are particularly useful for high frequency applications. However. for optimum coupling. the direction of mechanical stress must be identical with that of the piezo-electric axis and that of the alternating field; consequently transducers of this type are.

not readily applicable io audio frequency devices.

The minimum frequency at which such transducers may be operated is determined by the maximum capacitance, or electrical compliance, commensurate with practical values of mechanlI cal compliance. If the mechanical compliance is made large enough to Isuit practical requirements by making the cross-sectional area small. the associated capacitance is too small for audio frequency applications. If. on the other hand, a practical value of capacitance is obtained by making the cross-sectional area large, the mechanical compliance becomes inconvenlently smalL It is a particular object of this invention to provide an improved artificial piero-electric transducer which is suitable for operation at audio frequencies.

In accordance with the above-identified Adler application, it has been found that the capacitance or electrical compliance of thearticlal transducers may be increased while maintaining a practical value ot mechanical compliance by varying the direction of the piezo-electric axis within a single-unit transducer. It is a further object of the invention to provide a novel ceramic transducer, sensitive to bending stress, which utilizes this principle:

Frequent applications of piezo-electric trans- River Forest,- lll.l assigner to Radio Corporation, a corporation of application December 2e. 1947, serial No. this application May 2,

ducers in the audio frequency range are made in phonograph pickups, microphones, and the lure. In such applications, it is necessary to provide a.

transducer which has permanent piezo-electric properties with respect to bending stress. It is an important object of the present invention to provide a novel piezo-electric polycrystalline aggregate transducer. sensitive to bending stress. which is suitable for use as a phonograph pickup or the like, and which is rugged. inexpensive, and durable.

In the speciiication and claims, the term direction," as applied to the piezo-electric axis or to the polarizing iield,includes the concept of sense, therefore the phrase piezo-electric axes having di'erent directions" includes piezo-electric axes of opposite sense.

The term unidirectional,. as applied to polarizing fields, ls employed with reference to time. and not with reference to space, Hence a unidirectional" polarizing field is one which is produced by a unidirectionalpotential difference.

The terminology "polycrystalline aggregate" is employed to connote a unitary structure comprising a large number oi minute crystals. The term "ceramic" necessarily implies such a structure.

The features of the invention which are believed to be novel are set forth particularly in the appended claims. 'I'he invention may more readily be understood, however, by reference to the following description taken in connection with the accompanying drawings, in which like reference numerals indicate like elements, and in which:

Figure 1 is a schematic reprsentation ofV e theoretical means for attaining the objects set forth above.

Figure 2 is a schematic representation of a practical method lor approximating the theoretical optimum condition shown in Figure 1.

Figure 3 is an enlarged view of a. section of igure 2 showing the electrostatic aux distribu- Figure 4 is a schematic representation, partly in section. of a ceramic element sensitive to bending stress.

Figures 5 and 6 are schematic representations of a ceramic transducer sensitive to bending stress.

-Fieures 7-9 are schematicrepresentations of an embodiment constructed in accordance with the invention.

Figure l0 is a perspective view of a physical embodiment of the invention.

Referring now to Figure 1, there is shown schedotarse n ab matically a slab 20 of piezo-electric material. the direction of the piezo-electric axis being assumed to be longitudinal. It is seen that the mechanical compliance in the longitudinal direction may wellbe high enough to suit practical requirements, since the cross-sectional area is small into n slices 23--32 of equal length; the dotted lines indicate any desired number ci lntermedb ate slices. The capacitance between the imaginary transverse surfaces of each slice is therefore n times that between the two end surfaces 2l and 22 of slab 2li. while the mechanical compliance between surfaces 2i and 22 remains unchanged. Furthermore. by interconnecting al- 'tornate transverse faces of the imaginary slices ils-32. as shown, and by connecting each set of alternate transverse faces to one o' a pair of terminals 33 and Bil, the total capacitance between surfaces 2i and 22 is made equivalent to n times the capacitance between the end faces Nof an individual imaginary slice 28: consequently the eiective capacitance between surfaces ill and 22 is increased 'oy a factor of n.

Ideally such a condition may be accomplished by polarizing adjacent slices 23-32 along the same axis but in opposite directions. Such a condition is shown schematically by the arrows which indicate the. direction of polarization. and hence the direction of the piezo-electric axis, in each of the imaginary slices.

It has been found that permanent piezo-electric properties may be induced in certain polycrystalline aggregates. and furthermore, that the direction of the piezo-electric axis induced in such aggregates is identical to that o the electrostatic ilux set up by the polarizing elds. Consequently. such a condition as shown schematically in Figure l might be accomplished if suitable unidirectional polarizing nelds could be set up between the. transverse faces of each of the imaginary slices 2li-32. Obviously. such a situation is impractical, since no electrodes exist on the imaginary transverse surfaces between the slices.

As a practical approach to the optimum condition shown in Figure 1. there is shown in Fisure 2 a slab dll of suitable polycrystalline aggre gate. such as a ceramic comprising barium titanate or barium strontium titanate mined with a small amount of a glass forming oxide and red to vitrlfication in accordance with the aforementioned copending Cherry application. o'n opposite faces Il and l2 of which have been disposed a -plurality of electrical terminals or electrodes IIS-54. For purpose of illustration. twelve terminals have been shown: however. other numbers of terminals may be employed. Alternate pairs of opposite terminals d3, dd. 4l, IMEI. S2 and 45. 46. 69. 5D, 53. 5G are interconnected and brought out to a pair of input terminals 55 and 56 respectively. When a' unidirectional potential difference is applied between input terminals 55 and 5G. adjacent portions oi slab lll are polarized in substantially opposite directions.

In order more fully to show and explain such polarization. there is shown in Figure 3 a detail view of a portion oi slab Bil of Figure 2. The electrostatic ilux distribution, and hence the dllill recticn o1 polarization. is assumed. for purposes or explanation, to have a sense from to If then, it is assumed that input terminal EE is the positive terminal and input terminal 66 is the negative terminal, the electrostatic field, and hence the direction oi the piezo-electric axis dif- 1ers abruptly from portion to portion as schematically shown in `Pigure 3.

It is seen that the embodiment shown and` described in conjunction with Figures 2 and 3 is only an approximation of the ideal shown and described in conjunction with Figure 1. Since ,those portions oi' the slab i0 of ceramic material which lle between opposite pairs of terminals, dl and 48. for example, carry little or no electrostatic flux. these portions may be regarded as being waste portions." In order to effect a compromise between the desired effective increase in electrical compliance and a minimum of "waste material," it has been found that the width of the individual terminals 43-54 should be of the order of the thickness of the slab 40, and thedistance between successive terminals. lll and H9. for example. should be of the order of twice the thickness of the slab di). These proportions are the result oi practical experiment and are intended in no sense to be construed as limitations, as other proportions may be used with varying degrees oi eiliciency.

In certain audio frequency applications. it is desirable to employ a piezo-electric element which is sensitive to bending. Two elements. each Zormed in accordance with the method shown and described in connection with Figure 2, may be fastened together, by cement or in some other 'suitable manner, to provide such a dimorphic clement. Such a configuration is shown in Figure 4. wherein a pair of elements 6B and 6I are fastened together to form a composite body 62. In operation one element GEI is instantaneously subjected to tensile stress while the other ele ment 6I is instantaneously subjected to compressive stress, or vice versa. Therefore, care must be taken that elements Eil and Gi are as sembled and electrically connected in such a annex' that the useful outputs of the individual e ements are additive in the composite transduce'.

While a transducer having permanent piezoelectric properties with respect to bending stress may be produced in the manner shown and desibed in conjunction with Figure 4, a simplied transducer of this type may be provided. An element sensitive to bending stress may be provided by properly applying polarizing fields to a single unitary polycrystalline'aggregate body. Such an element is shown ln Figure 5. wherein a unitary slab 1li of barium titanate or other suitable aggregate is provided with a. plurality of electrical terminals 'ril-82 similarly disposed along opposite faces 81 and 8L Alternate terminals 1|, 15, and l! on race 83 and alternate terminals 14. 1l. and 82 on face 81B are interconnected and brought out to an input terminal 85. and the remalnder of the terminals 12, 13. 16, 11, Bn, and 8| are interconnected and brought out to a second input terminal 8B.

By applying a unldirectionlal potential difierence between input terminals 85 and 86. unidirectional polarizing iields are applied between successive terminals. 'il and 13. for example. on each tace Il. ll of slab 10. Furthermore, each termina! (1|, for example) is oppcsitely-polarized from the terminal (12, for example) which is directly opposite therefrom. and the polarizing fields between opposite pairs of successive ter-l minals (1i, 13 and 12, 1I for example) are opposite in sense. By these means. the direction or sense o! the piezo-electric axis is made to dliler abruptly from portion to portion throughout the single piece 1D in a manner similar to the piezoelectric axis distribution in a. composite dimorphic element such as that shown in Figure 4.

After maintaining the polarizing nelds lor a sumcient period of time at least to approach saturation of the piezo-electric eiiect, such fields are removed, and the connections of terminals' lI-Bl are changed to insure additive outputs in response to bending.V Slab i with terminals li-li reconnected for proper output at terminals l and I6 is shown in Figure 6, in which alternate pairs oi opposite terminals 1l. 12, 15, 1G, ll. and B0. and ll. 14, 11. ll, Il. and I2 are interconnected. Such an interconnection is desirable in order to minimize the shunt capacitance between output terminals Il and Il which etiec tively reduces the useful'output of such a transducer. The embodiment ot Figures 5 and 6 is spe cifically disclosed and claimed in the copenciing application of Walter L. Cherry, Jr., Serial No. 159,613, illed concurrently herewith, for Piezo- Electric Transducer and Method for Producing Same, and assigned to the present assignee.

Figure 'l shows an alternative method or producing essentially the same result while greatly reducing any shunt capacitance across the output terminals. In this embodiment, in accordance with the present invention. a pair of polycrystalline aggregate bodies 9i) and Il are fastened together by cementing means I2 of low dielectric constant thereby to .form a composite body. Cementing means l! may comprise a layer of cementing material of low dielectric constant, although the desired results may be achieved by glazing slabs 9U and Si and nring with the glazed sides in contact, by nring a sandwich" oi three ceramic slabs, or by other suitable means. Op

posite faces of the composite body are provided with a. plurality oi similarly disposed electrical terminals or electrodes 83-9! and "-404, a1- ternate pairs of opposite terminals 53. 09, l5, ill, 91, |03, and 94, |00. 8E, |02, 88. iol being interconnected and brought out to a pair oi' input terminals lili, and lili, respectively. After polarizing in the usual manner, the terminal connections on one face of the composite body are reversed to insure additive outputs in responsevto bending, as shown in Figure 8, and terminals ills and HIB become output terminals. Although opposite terminals (83 and I9, for example) have opposite polarities associated therewith, the thin layer of cementing material 9! so reduces the e1- fective capacitance shunting the output that satisfactory operation is insured.

While the embodiment shown in Figures l and 8 is formed by cementing bodies lil and Il together before polarization, it is to be understood that the individual bodies 80 and Il may be polarized separately and then ilrmly united insuch manner as to insure additive outputs in response to bending. Such a method is shown schematally in Figure 9, in which body lil is separately polarized. Alternate terminals 93, 95, ll and li, Si. ll are connected to respective input terminals |05 and |05. After polarization. two such bodies are cemented or otherwise fastened together to produce a composite body such as that shown in Figure 8.

It is also to be understood that, although pairs oi' terminals l-IN have been shown connected terminals may be avoided without the use oi low4 dielectric constant cementing means.

As a further variant, the composite body shown in Figures 'I and 8 may be polarized with the terminals 93--i04 connected as in Figure 8, in which case the terminals II-IM are reconnected as shown in Figure 7 to insure additive outputs. In this manner, the shunt capacitance across the output is minimized by interconnecting opposite terminals.

There is shown in Figure 10 a phonograph pickup lill constructed in accordance with the invention. The terminals lil and Il! are shown in the form of a pair of intermeshing'ccmbs for enlcient polarization, and may be oi silver paint or other suitable material applied by silk screen, vacuum evaporation, or other suitable process. In a practical application. the terminals applied on the back side (not shown) of element lill may be made in the form oi' a mirror image of-those applied to the front side in order to minimize the undesired capacitance shunting the output. One end Il! of transducer lill is rmly clamped in a bracket ill, and the other end I I5 is provided with a rigid extension IIS so constructed and arranged that the edges I Il and IIB of transducer lill and extension H6 all converge to a point I I9. Lateral motion at point H9, which may be translated from the unduiations of a record disk by means or a stylus ilil secured to extension IIB at point l i9, then results in corresponding electrical output between terminals lli and Ill; by making transducer -l l0 trapezoidal in shape, the conditions oi! a uniform-stress beam are appreached.

While there has been shown and described a. certain preferred embodiment oi the invention, it will be understood that numerous variations and modincations may be made, and it is .contemplated, in the appended claims, to cover all such variations and modifications as fall within the true spirit and scope oi the invention.

I claim:

l. The process of producing a permanently piezo-electric ceramic body sensitive to bending stress, said process comprising fastening together corresponding faces of a pair of polycrystalline aggregate bodies thereby to produce a composite body, similarly disposing along each of two opposite faces oi' said composite body a plurality of parallel electrodes, applying unidirectional polarizing fields having substantially opposite directions between successive pairs oi electrodes on each of said opposite faces, and maintaining said fields for a period of time sumcient at least to approach saturation of the piezo-electric effect in said composite body.

2. The process oi' producing a permanently piezo-electric ceramic body sensitive to bending stress, said process comprising fastening together corresponding faces cia pair oi' polycrystalline aggregate bodies by cementing means having a 7 body, similarly disposing along each of two opposite i'aces oi said composite body a plurality of parallel electrodes, applying polarizing fields having substantially opposite directions between successive pairs of electrodes on each of said opposite faces. and maintaining said fields for a period of time suiicient at least to approach saturation of the piezo-electric eillect said composite body.

3. The process or producing a permanently piezo-electric ceramic body sensitive to bending stress, said process comprising disposing along a single face of each of a pair oi similar polycrystalline aggregate bodies a plurality o paralicl electrodes, applying unidirectional polarizing fields having substantially opposite directions between successive pairs oi electrodes disposed along each of said faces. maintaining said fields for a period of time suttlcient at least to approach satu ration of the piezo-electric effect in said bodies. and firmly uniting the faces of said bodies which are opposite to the electrode-bearing'taces.

4. The process of producing a permanently piezo-electric ceramic body sensitive to bending stress, said process comprising disposing along a single face of each of a pair of similar polycryslline aggregate bodies a plurality of parallel electrodes, the disposition of electrodes on the second of said bodies being substantially a mirror image oi that on the rst oi' said bodies| applying unidirectional polarizing fields having substantially opposite directions between successive pairs of electrodes disposed along each of said faces. maintaining said elds for a period oi time sufficient at least to approach saturation o the piezo-electric eiect in said bodies, and firmly uniting the faces of said bodies which are opposite to the electrode-bearing faces.

5. A ceramic element having permanent piezoelectric properties. said element including a pair of solid polycrystalline aggregate bodies fastened together by cementing means having a dielectric constant which is low relative to that of said aggregate thereby to form a composite body, and a plurality ot electrodes similarly disposed along each of two opposite faces of said composite body, the direction of the piezo-electric axis being substantlally opposite between successive pairs of electrodes on each of said faces.

6. A ceramic element having permanent piezoelectric properties. said element including a pair of solid polycrystalline aggregate bodies fastened together by cementing means having a dielectric constant which is low relative to that of said asgregate thereby to form a composite body. said aggregate including individual crystals of barium titanate bonded together with a ceramic binder, and a plurality oi electrodes similarly disposed along each of two opposite faces of said composite body, the direction ofthe piezo-electric axis being substantially opposite between successive pairs of electrodes on each of said faces.

ALEXANDER ELLETT.

REFERENCES CUTE@ The followingreferences are of record in the ile 0f this patent:

UNITED STATES PATENTS Number Name Date 2,242,756 Pope May 20, i961 2,486,560 Gray Nov. 1, 1%9 

